/*
 * Copyright (c) 2013, SRI International
 * All rights reserved.
 * Licensed under the The BSD 3-Clause License;
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 * 
 * http://opensource.org/licenses/BSD-3-Clause
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 
 * Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 * 
 * Neither the name of the aic-expresso nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 
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 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 
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 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package com.sri.ai.grinder.library.controlflow;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import com.google.common.annotations.Beta;
import com.sri.ai.expresso.api.Expression;
import com.sri.ai.expresso.helper.Expressions;
import com.sri.ai.grinder.api.RewritingProcess;
import com.sri.ai.grinder.core.AbstractRewriter;
import com.sri.ai.grinder.core.FunctionApplicationProvider;

/**
 * An atomic rewriter for conditional expressions. Returns the then or else
 * branches directly if condition is trivial. Includes related helper methods.
 * 
 * @author braz
 * 
 */
@Beta
public class IfThenElse extends AbstractRewriter {

	public static final String FUNCTOR = "if . then . else .";
	//
	private static final List<Integer> _pathToFunctor   = Collections.unmodifiableList(Arrays.asList(FunctionApplicationProvider.INDEX_OF_FUNCTOR_IN_FUNCTION_APPLICATIONS));
	private static final List<Integer> _pathToCondition = Collections.unmodifiableList(Arrays.asList(0));
	private static final List<Integer> _pathToThen      = Collections.unmodifiableList(Arrays.asList(1));
	private static final List<Integer> _pathToElse      = Collections.unmodifiableList(Arrays.asList(2));

	@Override
	public Expression rewriteAfterBookkeeping(Expression expression, RewritingProcess process) {
		if (isIfThenElse(expression)) {
			if (expression.get(0).equals(true)) {
				return expression.get(1);
			}
			if (expression.get(0).equals(false)) {
				return expression.get(2);
			}
		}
		return expression;
	}
	
	public static List<Integer> getPathToFunctor() {
		return _pathToFunctor;
	}
	
	public static List<Integer> getPathToCondition() {
		return _pathToCondition;
	}
	
	public static List<Integer> getPathToThen() {
		return _pathToThen;
	}
	
	public static List<Integer> getPathToElse() {
		return _pathToElse;
	}

	/**
	 * Make an if then else expression, returning the then or else branches directly if condition is trivial.
	 */
	public static Expression make(Expression condition, Expression thenBranch, Expression elseBranch) {
		if (condition.equals(true)) {
			return thenBranch;
		}
		if (condition.equals(false)) {
			return elseBranch;
		}
		if (thenBranch.equals(true) && elseBranch.equals(false)) {
			return condition;
		}
		Expression result = Expressions.make(FUNCTOR, condition, thenBranch, elseBranch);
		return result;
	}

	public static boolean isIfThenElse(Expression expression) {
		boolean result = expression.hasFunctor(FUNCTOR);
		return result;
	}
	
	/** Returns the condition of an if then else expression. */
	public static Expression getCondition(Expression expression) {
		Expression result = expression.get(0);
		return result;
	}
	
	/** Returns the then branch of an if then else expression. */
	public static Expression getThenBranch(Expression expression) {
		Expression result = expression.get(1);
		return result;
	}
	
	/** Returns the else branch of an if then else expression. */
	public static Expression getElseBranch(Expression expression) {
		Expression result = expression.get(2);
		return result;
	}

	/**
	 * Make a copy of a given if then else condition, but for a replaced condition,
	 * possibly simplifying it if new condition is true or false.
	 */
	public static Expression copyWithReplacedCondition(Expression ifThenElse, Expression newCondition) {
		if (newCondition.equals(Expressions.TRUE)) {
			return getThenBranch(ifThenElse);
		}
		if (newCondition.equals(Expressions.FALSE)) {
			return getElseBranch(ifThenElse);
		}
		Expression result = IfThenElse.make(newCondition, getThenBranch(ifThenElse), getElseBranch(ifThenElse));
		return result;
	}
}
